1. Field of the Invention
The present invention relates to a method for pneumatically discharging hydromechanically conveyed hydraulic building material for underground operations according to the introductory part of claim 1. The invention also relates to an apparatus for carrying out this method.
2. Description of the Prior Art
Hydraulic building materials which are used underground are substances with consistencies ranging from granular to powderized and having different water/solid factors, which are frequently processed with aggregates of synthetic material or fiber mixtures, among other things, during gunning. The invention relates in particular to gun-applied concrete made of these materials, or gun-applied mortar, which in turn is appled several centimeters thick to the stone walls of underworkings, in particular of drifts, leaving out the floor, as soon as possible after the roof fall has been won, for example by blasting, in order to increase the self-supporting capacity of the surrounding ground. In addition to this roof fall protection when driving rooms in mining and tunnel construction, the inventive method also serves the purpose of sealing fire and ventilation dams and smoothing walls in order to reduce air resistance, as well as for timbering in general. Activator, which is preferably liquid, may be added to the building material in the interests of early strength, in order to guarantee optimal bearing capacity after as short a time as possible to keep down the convergence of the rock stratification, among other things.
The hydromechanical conveyance of the wet building material, in particular in the form of mortar or concrete, is advantageous as compared to dry conveyance, which is also known, in the case of which the necessary mixing water and possibly the activator are added to the building material at the end of the pipeline, in that the applied layers have a uniform composition in accordance with a given recipe, which eliminates the fluctuations in the solidity of the applied layers resulting from uneven composition of the building material and uncontrolled addition of water. The invention therefore assumes a previously known method by which the building material is discharged by aid of a mouth piece, provided with a nozzle, of a conveying pipeline or tube assembly acted upon by a pump, and gunned onto the surface to be coated.
In the case of the previously known apparatus, the building material is conveyed hydromechanically in the axial direction of the nozzle. A short distance before the nozzle, compressed air is added to the hydromechanically conveyed stream via nozzle channels arranged radially in the mouth piece. The resulting acceleration of the building material is subject to limits, however. For the nozzle device must take the limited compressibility of the wet building material into consideration due to the danger of clogging. The hydromechanical conveyance of the building material has an aggravating effect on this when the building material, for example due to the reduced friction in the conveyor pipe, is conducted in the latter with a relatively large cross-section and cannot be supplied simultaneously by a distributor to a number of discharge pipes or tubes. For in such cases the hydromechanically conveyed stream must also be reduced to a smaller cross-section at the mouth piece, which is determined by the handling of the mouth piece with the necessary strength of an operator, provided manipulators or monitors cannot be used.
Since, regardless of these difficulties, the activator is preferably added only at the last minute due to the danger that the hardening building material might interfere with the paths of hydromechanical conveyance, this is done with one or more nozzles at the end of the hydromechanical conveyance path. However, this means that the activator does not mix completely and homogeneously with the hydraulic building material. The produced layers are therefore inhomogeneous where the required early strength is not attained. Furthermore, there are also losses of activator liquid which is then carried along by the radially directed air blast and leads to concentrations of noxious substances in the atmosphere.
This, along with other causes, may lead to rebound losses, i.e. a percentage of discharged building material which does not stick, but falls down. It is true that the percentages ranging from 30% to 40% which are usual in dry methods are not reached by the wet methods, but the quota of the latter has different causes. It depends, among other things, on the adhesive power of the building material, the angle of impact of the building material stream discharged from the mouth piece, and similar parameters. In particular, the systematic changes in the supporting capacity of the substratum hit by the gunned building material constitute one of the most important causes for rebounding. For regardless of the force of impact, for example, on a rock surface, the resistance of the substratum changes in the course of the application of the gunning layer and is generally lower the more the applied layer thickness increases. The early strength of the building material thus plays as much of a part in this connection as the quantity of building material discharged in each particular case. But the known apparatus does not allow for any control of the speed of impact or any use of activator to avoid excess rebound losses, at least not to the necessary extent.